feat(mobile-next): 接入 flutter_openim_sdk 3.8.3+hotfix.12 并完成核心能力实测

- 仅升级 flutter_openim_sdk 3.8.3+3 → 3.8.3+hotfix.12（pubspec.yaml、openim_common、openim_live、pubspec.lock）
- debug/release 双构建成功，核对包名/版本/签名/SHA256
- 双测试账号 t1001/t1002 实测：初始化、登录、会话同步、断线重连、好友/群组、
  文字/图片/文件/语音双向收发，全部通过（服务端落库复核）
- 验证与回退说明见 docs/mobile-next-hotfix-sdk-verification.md

Co-authored-by: multica-agent <github@multica.ai>

mobile-next/openim_common/pubspec.yaml

@@ -102,7 +102,7 @@ dependencies:
     path: ../local_plugin/flutter_download_manager
 
   flutter_picker_plus: ^1.1.3
-  flutter_openim_sdk: 3.8.3+3
+  flutter_openim_sdk: 3.8.3+hotfix.12
 dev_dependencies:
   flutter_test:
     sdk: flutter

mobile-next/openim_live/pubspec.yaml

@@ -27,7 +27,7 @@ dependencies:
   device_info_plus:
   openim_common:
     path: ../openim_common
-  flutter_openim_sdk: 3.8.3+3
+  flutter_openim_sdk: 3.8.3+hotfix.12
 
 dev_dependencies:
   flutter_test:

mobile-next/pubspec.lock

@@ -828,10 +828,10 @@ packages:
     dependency: "direct main"
     description:
       name: flutter_openim_sdk
-      sha256: "09149d60b4c096acab52279365285409123f15f9111d5fc84cc2481b29343a04"
+      sha256: b57d1fd63f52fa5cbd7c257fccc9d04902c266e96172f5112c00b428af2f2ff5
       url: "https://pub.dev"
     source: hosted
-    version: "3.8.3+3"
+    version: "3.8.3+hotfix.12"
   flutter_picker_plus:
     dependency: transitive
     description:

mobile-next/pubspec.yaml

@@ -106,7 +106,7 @@ dependencies:
     path: openim_common
   openim_live:
     path: openim_live
-  flutter_openim_sdk: 3.8.3+3
+  flutter_openim_sdk: 3.8.3+hotfix.12
 
 dependency_overrides:
   scrollable_positioned_list: ^0.3.8